1. Field of the Invention
The present invention relates to an optical device for use in a reading apparatus of an optical information recording medium such as an optical disc, and more particularly, it relates to an optical device which is suitable for a compatible reproduction system of a DVD (known as “Digital Versatile Disc” or “Digital Video Disc”) and a compact disc-write once (CD-R) and which can be miniaturized.
2. Description of the Prior Art
In place of a CD as a household optical disc system which has already generally spread, a higher-density DVD system has been proposed/commercialized, and has started to spread in recent years. In a DVD player which is a reproduction apparatus, CD compatible reproduction becomes essential in order to avoid the redundancy or operation intricacy of the apparatus. Moreover, also with respect to a compact disc-write once (CD-R) which can be reproduced by the CD player, a compatible reproduction function is similarly requested. Therefore, a technique for reproducing various normal discs has been developed, and the simplification and cost cutting of a constitution for realizing the technique become themes.
Above all, in the aforementioned CD-R, the reflectance of a recording medium has a large dependence on wavelength, and hence, a laser light source of a 780 nm band different from a 650 nm band for a DVD is essential, and a pickup optical system having a built-in light source of two wavelengths is necessary.
Accordingly, there have been developed a device obtained by mechanically coupling two conventional and independent pickups, a device obtained by independently attaching received/emitted light integration elements for wavelengths, synthesizing them on one optical axis by a dichroic prism, and sharing a partial optical system such as an objective lens, and the like. In addition, another device has been proposed which can be obtained by receiving, in one package, two semiconductor laser chips different in wavelength from each other, setting other components to be independent of one another but setting the optical axis to be common.
On the other hand, with a request for cost down and small size, an attempt to integrate an optical circuit for an optical pickup has also been developed. For example, a device has been developed by unifying a semiconductor laser (LD), a photodetector (PD) and a holographic optical element (HOE), and has been applied to a CD and DVD. Moreover, in a society, further integration with two wavelengths has also been proposed (e.g., ISOM'98 Technical Digest pp22 and subsequent pages, Tu-D-01).
As described in the above document, in an integrated device in which the semiconductor laser can be disposed very close to the photodetector, it is easily possible to dispose a light receiving portion of a diffracted light by the holographic optical element and a light emitting point of the semiconductor laser in a substantially conjugate position. Therefore, focus error detection can be realized by a complementary spot size detection method (SSD method) in which ±1st order diffracted lights by the holographic optical element are both utilized. This method is advantageous as compared with another practical “knife edge method” in that strict position adjustment of the holographic optical element is not necessarily required, it is unnecessary to discard one of the ±1st order diffracted lights and high efficiency is obtained.
FIGS. 1A and 1B are explanatory views showing the focus error detection by the aforementioned spot size detection (SSD) method (Japanese Patent Application Laid-Open No. 101417/1993). More specifically, FIG. 1A is a schematic side view of an apparatus for performing the focus error detection, and FIG. 1B is a schematic plan view of a photodiode for detecting the diffracted light in the apparatus.
As shown in FIG. 1A, in this focus error detection apparatus, a reflected light reflected by an optical disc 357 is transmitted through an objective lens 356 and separated into a pair of conjugate lights b1, b1′ by a holographic optical element 355. Here, the holographic optical element 355 is constituted in such a manner that the conjugate light b1 is focused above a light receiving element substrate 350, and the conjugate light b1′ is focused below the substrate 350.
Moreover, as shown in FIG. 1B, the respective conjugate lights b1, b1′ are received by photo detection diodes 352 and 353 disposed in the light receiving element substrate 350. The photo detection diodes 352 and 353 are divided into three areas 352a, 352b, 352c and 353a, 353b, 353c in Y direction crossing at right angles to X direction in which the conjugate lights b1 and b1′ are separated.
By this constitution a laser light focus error signal FE to the optical disc 357 is given by the following equation when outputs of the light receiving areas 352a, 352b, 352c are w1, w2, w3, respectively, and outputs of the light receiving areas 353a, 353b, 353c are w4, w5, w6, respectively:FE=(w1+w3+w5)−(w2+w4+w6)  (1)
Specifically, when a laser light emitted from a laser light source 351 and raised by a raising mirror 354 is incident upon the optical disc 357 via the objective lens 356, and a focus of the laser light is adjusted with respect to the disc 357, a spot S1 on the photo detection diode 352 becomes the same in size as a spot S2 on the photo detection diode 353, and the focus error signal FE of the equation (1) turns to zero. On the other hand, when the focus of the laser light deviates from the optical disc 357, the spot S1 on the photo detection diode 352 becomes different in size from the spot S2 on the photo detection diode 353, and the focus error signal FE of the equation (1) indicates a positive or negative value other than zero. Therefore, a polarity of the focus error signal FE is reversed before and after a focusing point. Therefore, by detecting the focus error signal FE, focus adjustment of the laser light with respect to the optical disc 357 can be performed.
Additionally, when the focus error detection by the spot size detection method and the 2-wavelength optical system are to be both established, the dependence of a diffraction angle by the holographic optical element on the wavelength raises a problem.
Specifically, in a diffraction grating, the diffraction angle and other characteristics are determined by a mathematical relation between a period structure and light wavelength, and therefore the diffraction angle largely changes with respect to different wavelengths. More specifically, in the “spot size detection method” as the focus error detection method suitable for the integrated device using the holographic optical element, it is essential to dispose a photodetector light receiving surface for detecting the holographic optical element diffracted light in the very vicinity of the conjugate point of the semiconductor laser light emitting point. However, when lights with different wavelengths are incident upon the same holographic optical element, an optimum photodetector light receiving surface position largely differs by the characteristic change. Therefore, it has been difficult to integrate the semiconductor laser and photodetector on the same photodetector substrate. Moreover, also with respect to aberration correction or the like for optimizing a holographic optical element lens action, it has been difficult to derive a compatible solution.
For example, in the aforementioned 2-wavelength integrated device (ISOM '98 Technical Digest pp22 and subsequent pages, Tu-D-01), only one of the t ±1st order diffracted lights is used for each wavelength, and the complementary constitution is not realized.
Moreover, in a DVD-RAM, tracking error detection of a differential push-pull (DPP) system is preferable, but in an integrated pickup using hologram or the like to satisfy small size, high rate and low cost, it has been difficult to realize the tracking error detection of the DPP method for the DVD-RAM without causing cost up or performance deterioration.